THE UNIVERSITY OFWESTERN AUSTRALIA
School of Mechanical Engineering
Computational Fluid Dynamics MECH4406Major Project: Improvement of Flow through
the Implementation of a Bellmouth
Completed by: Jack Lu 10426723
Lecturer: Phillippa O’NeillDue Date: Wednesday 30th May 2007
CFD MECH4406 Major Assignment Jack Lu 10426723
Contents
1.0 INTRODUCTION.............................................................................................................................3
1.1 ABSTRACT...................................................................................................................................3
1.2 DESIGN OVERVIEW..................................................................................................................3
2.0 GEOMETRY MODEL......................................................................................................................4
3.0 CFD MESHING................................................................................................................................6
3.1 Defining Regions...........................................................................................................................6
3.2 Mesh Sizing...................................................................................................................................7
3.2.1 Body Spacing..........................................................................................................................7
3.2.2 Face Spacing...........................................................................................................................7
3.3 Creating the Mesh..........................................................................................................................8
4.0 CFD MODELLING IN CFX-PRE..................................................................................................10
4.1 Domain.........................................................................................................................................10
4.2 Physics.........................................................................................................................................10
4.2.1 Simulation Type....................................................................................................................10
4.2.2 Fluid Models.........................................................................................................................10
4.3Boundary Conditions....................................................................................................................11
4.3.1 Air Opening Boundary Condition.........................................................................................11
4.3.2 Outlet Boundary Condition...................................................................................................11
4.3.3 Physical Boundary Condition...............................................................................................11
4.4 Write to Solver.............................................................................................................................11
5.0 CFD SOLVING IN CFX-SOLVER................................................................................................12
6.0 CFD ANALYSIS IN CFX-POST....................................................................................................14
6.1 Straight Pipe Intake......................................................................................................................14
6.2 Bellmouth Intake..........................................................................................................................17
CONCLUSION......................................................................................................................................21
REFERENCES......................................................................................................................................22
APPENDIX............................................................................................................................................22
APPENDIX Bellmouth A..................................................................................................................22
APPENDIX Bellmouth B..................................................................................................................24
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1.0 INTRODUCTION
1.1 ABSTRACT
What do turbo chargers, air boxes and jet turbines have in common? They are all applications
that involve an intake pipe opening that draws air from a volume of air that is far greater than
that of the size of the pipe. Almost all practical applications today look for greater
efficiencies even in the smallest areas like the geometry of an intake pipe. Over the years it
has become common knowledge (particularly in motorsports) that fluid flow into an intake
pipe can be improved by adding a bellmouth (also known as a trumpet intake) to the end of
the intake pipe. This paper will investigate into the effects of bellmouth geometries using 3D
Computational Fluid Dynamics (CFD) analysis to optimise flow in the intake runners of a
Honda CBR600RR engine employed by UWA Motorsport. With CFD, it is possible to
achieve a good design without having to build multiple prototypes and experimentally test
each one of them. Ultimately this will lead to a considerable reduction in cost and time.
1.2 DESIGN OVERVIEW
The design of an ‘optimum’ intake runner will be a process of understanding flows and
pressure drops through the use of CFD packages such as ANSYS CFX. In order to utilise
ANSYS CFX other software packages such as SolidWorks or ANSYS Design Modeller is
needed in order to provide geometries for ANSYS CFX-Mesh to create a volume mesh upon.
The following software packages were used in this study.
Geometry & Modelling SolidWorks
Mesh Creation ANSYS CFX-Mesh & GAMBIT
CFD Setup ANSYS CFX-Pre & FLUENT 6
CFD Solving ANSYS CFX-Solver & FLUENT 6
CFD Analysis ANSYS CFX-Post & FLUENT 6
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CFD MECH4406 Major Assignment Jack Lu 10426723
As this study will be looking into increasing the performance of the UWA Motorsport intake
package, it will be wise to simulate plenum conditions in terms of pressures. Undoubtedly
over complications will arise when an operating engine is included in the simulation as it will
introduce pressure waves which will deem the simulation transient but because this study is
only interested in the effects of the bellmouth opening it is reasonable to define this
simulation as steady state.
2.0 GEOMETRY MODEL
Before any CFD processing can be done, a geometry model must be constructed. The use of
SolidWorks in this application was chosen for its ease in modelling. ANSYS CFX-Mesh
creates a volume mesh in which holds the fluid domain. This means that the model created
prior to importing into CFX-Mesh must be an inverse of the initial geometry. In this case a
cube is created with the dimensions of 500mm3 holding a volume of 125L. A thin feature
revolve cut of the sketch seen in Figure 1 provides the boundaries of the pipe. The
dimensions in Figure 1 were chosen as an initial guess. The diameter of the pipe is 36.80mm
as this is the diameter of the intake ports on a Honda CBR600RR engine. The geometries
should be kept as simple as possible to reduce computing time so any minor details like fillets
and blends should be ignored when processing larger CFD models.
Extra time permitted this study to be compared to the results from FLUENT so the geometry
was modelled in 2D in GAMBIT. Modelling in GAMBIT deemed to be much more
complicated than SolidWorks as Cartesian coordinates must be inputted and connected
together to produce the geometry.
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CFD MECH4406 Major Assignment Jack Lu 10426723
Figure 1 Sketch of the revolve cut out
The reason why the box is so much larger than the pipe is to maintain uniform flow in areas
far from the intake pipe. The final geometry is a solid cube when viewed from the outside
with the revolve cut in the bottom centre as seen in Figure 2. The final file is exported as a
parasolid (*.x_t) as ANSYS CFX-Mesh is capable of importing parasolids.
Figure 2 Geometry model of the CFD fluid domain
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CFD MECH4406 Major Assignment Jack Lu 10426723
3.0 CFD MESHING
In order to calculate the fluid flow within the domain to a reasonable accuracy, a good quality
mesh needs to be constructed. If the mesh is too fine it will result in a much higher
computational time than needed to achieve the same results if the flow is simple. The first
step in CFD meshing is to import the parasolid file into ANSYS CFX-MESH. CFX-Mesh
utilises a tetrahedral mesh seen in Figure 3 for the mesh construction. This type of mesh is
totally unstructured and is capable of evenly distributing cell spacing in areas where the flow
insignificant (far away from the detail) and compacting cell spacing in areas of high detailed
flow (near boundary layers). The advantage of having this type of unstructured mesh is that it
allows for maximum flexibility in matching mesh cells with the boundary surfaces and for
putting cells where you want them (Anderson 1995). The only length scale mesh control
employed in this study is the body spacing and face spacing controls which is discussed in
Section 3.2.
Figure 3 Triangular surface mesh
3.1 Defining Regions
2D Regions in the model are defined in CFX-Mesh for the location of a meshing feature.
There are three main regions of concern for this type of model. The first is the ‘air region’
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CFD MECH4406 Major Assignment Jack Lu 10426723
that is situated on all six faces of the cube and second is the ‘port region’ which is the
circular face situated on the underside of the cube. CFX-Mesh initially defines all faces in the
model as a ‘Default 2D Region’ until the user has manually defined a face as a 2D region.
This means that the last region of concern being the walls of the pipe is left under the title of
‘Default 2D Region’.
3.2 Mesh Sizing
3.2.1 Body Spacing
The body spacing controls the mesh length scale of the volume. It is the initial length scale
before any face spacing or any other length scale control is applied. There is only one
parameter in this option which is the maximum (body) spacing allowed. For this study a
maximum body spacing of 15mm was chosen to achieve a fine mesh for more accurate
results. More refined body spacing would lead to a higher computing time and since the flow
being modelled is considered simple the 15mm maximum spacing seemed reasonable.
3.2.2 Face Spacing
The face spacing controls the mesh length scale on a face (or faces) and in the volume
adjacent to that face. There are four types of face spacing that can be chosen and for this
model angular resolution was chosen.
a. Angular Resolution
b. Relative Error
c. Constant
d. Volume Spacing
Edge lengths on particular faces can be varied upon the local curvature using the angular
resolution face spacing type. This allows for shorter edges to be used upon highly curved
faces and longer edges on flatter surfaces. It is controlled by the Angular Resolution
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CFD MECH4406 Major Assignment Jack Lu 10426723
parameter which will determine φ in Figure 4. For this study an angular resolution of 15o is
employed due to the high amount of curvature in the model’s geometry.
Figure 4 Angular Resolution node spacing based upon angle φ (ANSYS 2007)
The next step in the face spacing step is to set the minimum and maximum edge lengths.
These are the minimum and maximum edge lengths of the triangle meshes and ultimately
define the mesh cell size. Again the smaller the cells the higher the computational time
required can be and the larger it is a more inaccurate solution can result. The minimum edge
length for this study is set to 0.5mm whereas the maximum was set to 15mm. A larger
minimum length could have been set to achieve the same results but even with the 0.5mm
minimum length the difference in computational time required was not of great deal.
3.3 Creating the Mesh
Once the regions were defined and the mesh spacing set the mesh is ready to be created. A
surface mesh can be previewed to ensure that the mesh size is adequate for the simulation by
selecting the ‘Generate the surface mesh for the current problem’ button under the CFX-
Mesh main toolbar. When ready the complete volume mesh can be created as a *.gtm file by
selecting ‘Generate volume mesh for the current problem’ button.
The final mesh produces approximately 420 000 elements across all of the meshes produced
for the different bellmouths in this study. Calculating time for each simulation was
approximately 20 minutes each which is considered reasonable for the refined mesh created.
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CFD MECH4406 Major Assignment Jack Lu 10426723
For FLUENT a mesh was created in GAMBIT and like ANSYS CFX-Mesh has an advantage in
the refinement of the mesh and in this case, a higher density of triangle cells was placed in
areas of interest as seen in Figure 5. The only trade off is the user-friendly interface that CFX-
Mesh holds but GAMBIT seems to be a much more powerful tool if utilised properly.
Figure 5 Refined mesh allowed higher density of triangles in the areas of higher detail
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CFD MECH4406 Major Assignment Jack Lu 10426723
4.0 CFD MODELLING IN CFX-PRE
To set up the simulation CFX-Pre is employed to define the properties of the flow. To begin
the mesh is imported into CFX-Pre as an assembly using the Quick Setup option in CFX-Pre.
The convergence criteria can be set to default as it is sufficient to provide solutions for the
simple simulation. The simulation properties that were modified from default are explained
below.
4.1 Domain
The Domain options allow the user to edit the domain, type of domain, fluid, reference
pressure and other options. The domain that was imported is a fluid domain that is defined as
air at 25oC. In the UWA Motorsport plenum air temperatures are slightly higher due to heat
transfer from the combustion chamber but for the sake of simplification air properties at 25oC
are suffice. The reference pressure is left at 1atm as this simulates external atmospheric
conditions.
4.2 Physics
4.2.1 Simulation Type
In the UWA Motorsport intake plenum pressure waves in transient conditions complicate the
simulation so to keep the simulation simple the simulation type was chosen to be steady state
as we are only interested in the flow occurring around the bellmouth.
4.2.2 Fluid Models
No heat transfer model is selected again to reduce the number of calculations needed. The
turbulence model selected by default was the k-Epsilon model as it is suffice to produce the
results needed in this simulation. Again for the sake of simplicity, other options were left to
default.
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CFD MECH4406 Major Assignment Jack Lu 10426723
4.3Boundary Conditions
Three boundary conditions were set for this simulation and are explained below.
4.3.1 Air Opening Boundary Condition
The air boundary is linked to the ‘air region’ and is set to an opening with a relative pressure
of -20kPa. The opening type was selected over inlet type because it allowed air to flow both
ways as it if were open to atmosphere to ensure uniform flow occurs away from the intake
pipe. The relative pressure of -20kPa is used to simulate the internal plenum pressure when
the engine is operating at low speeds. The turbulence intensity remained default at 5% as
we’re trying to maintain a constant flow in areas away from the pipe.
4.3.2 Outlet Boundary Condition
The outlet boundary condition is linked to the ‘port region’ and is set as an outlet with an
average static relative pressure of -80kPa. This relative pressure is an average over the whole
outlet to allow for any variations. -80kPa was chosen as an estimation using the compression
ratio of the engine (12:1) with the consideration of valve overlap. It is important to note that it
is set as a subsonic outlet but in reality velocities are capable of reaching or exceeding Mach1
within the runners of the intake plenum particularly at higher engine speeds.
4.3.3 Physical Boundary Condition
If no other region is set as a boundary condition manually, CFX-Pre sets it as a physical no
slip smooth wall boundary condition by default which is good enough for this simulation.
4.4 Write to Solver
Once all preliminary settings were defined the simulation can begin first by saving the CFX-
Pre definitions as a *.def file that can be read by the solver.
The FLUENT simulation was set up with the same parameters as that of ANSYS CFX-Pre
above.
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CFD MECH4406 Major Assignment Jack Lu 10426723
5.0 CFD SOLVING IN CFX-SOLVER
Once the definition file has been loaded, the solver is capable of solving the simulation.
The solver uses the Navier-Stokes equation which are:
Continuity (conservation of mass)
Momentum (non-conservation form)
For this simulation, it took approximately 20 minutes over 56 iterations to complete
producing an output file with the simulation’s log and plots an updated residuals graph seen
in Figure 6.
FLUENT took approximately 30 seconds to solve the 2D case with 144 iterations and the
residual plot can be seen in Figure 7. The time taken in FLUENT is far less than ANSYS CFX
as FLUENT only analyses the 2D case.
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CFD MECH4406 Major Assignment Jack Lu 10426723
Figure 6 CFX-Solver Output with residual graph on the left and process log on the right
Figure 7 Residual plot from FLUENT 6
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CFD MECH4406 Major Assignment Jack Lu 10426723
6.0 CFD ANALYSIS IN CFX-POST
CFX-Solver also puts out a results file in the form of *.res in which requires CFX-Post to
open. In CFX-Post a plane is created upon the XY plane so that contour plots can be created.
Contour plots provide a good indication of what is occurring as the bellmouth geometry is
symmetrical 360o all around the vertical axis. It is important to note when comparing colour
contour plots, the legends need to be standardised so that a true comparison can be observed
between candidates.
6.1 Straight Pipe Intake
The first case investigated was the straight pipe intake and using CFX-Post a velocity vector,
u-velocity, and pressure contour plot were taken. The outline in Figure 8 shows flow
separation and some circulation occurring as U-velocities pass around the straight bend. Flow
separation is not sort after because the flow becomes detached from the surface of the object
and instead takes form of eddies and vortices which can restrict flow (Ravinder & Prasad
2006). Note that as predicted earlier, the maximum velocity achieved through the pipe is
358m/s, which is greater than Mach1 approximately 20mm downstream from the entry.
The U-velocity plot in Figure 10 shows high velocities at the tip of the pipe which can be
used to explain the flow separation occurring 20mm below it. The straight pipe is
experiencing a maximum horizontal velocity of 198m/s and this velocity must be greatly
reduced to prevent flow separation from occurring.
Lastly the pressure plot in Figure 11 shows poor pressure recovery from the sudden
contraction. It shows the pressure slowly increasing downstream of the pipe from the huge
pressure drop located on the walls of the entry of the pipe. This means that a longer runner
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CFD MECH4406 Major Assignment Jack Lu 10426723
length will be needed for pressure recovery and this may not be ideal in terms of packaging
or ideal for tuned runner lengths.
Figure 8 Velocity contour plot with velocity vectors for a straight pipe intake
Figure 9 Velocity countour plot along the straight pipe15
CFD MECH4406 Major Assignment Jack Lu 10426723
Figure 10 U-velocity contour plot for a straight pipe intake
Figure 11 Pressure contour plot for a straight pipe intake
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CFD MECH4406 Major Assignment Jack Lu 10426723
6.2 Bellmouth Intake
Three different cases of bellmouth designs were investigated each with their own unique
curvature. Problems observed with the flow into the straight pipe were mainly identified to be
flow separation and the poor pressure recovery. Any bellmouth geometry immediately
corrects both problems but to what extent is the matter. To view the effects of other
bellmouth geometries please see the Appendix. The chosen bellmouth’s geometry can be
viewed in Figure 1.
Flow separation problems no longer exists as seen in Figure 12 which immediately reduces
the likelihood of creating eddies and assists in the sought after laminar flow within the pipe.
This result is reinforced with a re-run of the simulation in FLUENT producing a velocity
vector plot seen in Figure 16. The U-velocity plot in Figure 14 also shows a large velocity
reduction in the horizontal plane which further reinforces the relationship between the U-
velocity and flow separation in this case as there was no flow separation present.
Secondly the maximum velocities experienced at the entry of the intake pipe has significantly
reduced from 358m/s to 290m/s and overall downstream of the pipe, remains quite constant
whereas the straight pipe dramatically loses its velocity to approximately 270m/s. In terms of
engine performance, this will play a large factor in increasing the engines volumetric
efficiency as higher velocities will mean a higher momentum since momentum p=mv.
The pressure recovery from the bellmouth design is seen to be a large improvement over that
of the straight pipe (see Figure 15). A large drop in pressure occurs on the side walls of the
pipe just as the cross sectional area of the pipe dramatically reduces. Pressures become
uniform approximately 10mm downstream from that area compared to the gradual pressure
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CFD MECH4406 Major Assignment Jack Lu 10426723
changes along the straight pipe which explains the much more laminar flow experienced in
Figure 12.
The velocity magnitude outputted by FLUENT identified from the vectors colour in Figure 16
shows a slight disagreement in the velocity as it is seen to have a slightly higher magnitude in
the areas of low pressure seen in ANSYS’ pressure plot of Figure 15. This is believed to be
the correct case as it is expected a velocity increase in areas of low pressure.
Figure 12 Velocity contour plot with velocity vectors showing no sign of flow separation
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CFD MECH4406 Major Assignment Jack Lu 10426723
Figure 13 Velocity contour plot along the bellmouth pipe
Figure 14 U-velocity plot showing reduced velocities in the horizontal plane
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CFD MECH4406 Major Assignment Jack Lu 10426723
Figure 15 Pressure contour plot showing constant pressure within the pipe
Figure 16 FLUENT velocity vectors shows similar results to ANSYS CFX
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CFD MECH4406 Major Assignment Jack Lu 10426723
CONCLUSION
The results seen were expected in terms of the improvements by implementing the bellmouth
but from this study I have gained a better understanding why. The effect of employing this
geometry decreases the U-velocities (in the horizontal plane) at the intake and reduces or
eliminates flow separation. This produces an improved laminar flow downstream of the pipe
through better pressure recovery compared to that of the straight pipe.
I have also gained a better understanding of how CFD operates and what parameters were
important to be able to simulate and analyse a simple geometry. It has increased my
confidence in the ability to comprehend the effects of geometries on fluid flow and therefore
assist in my designs for the intake of the UWA Motorsport car. I have also learnt a great deal
on how to use the ANSYS and FLUENT packages for the study of CFD.
If there were more time, further studies could be allocated to the effects of implementing the
bellmouths on all four runners within a plenum to see further improvements. Because the
intake of the UWA Motorsport car draws air from a 20mm restrictor, it is wise to use CFD to
analyse the relationship between cylinder filling and the position of the 20mm restrictor.
Other ideas include a vector analysis within the plenum to reduce any recirculation by adding
baffles. There are so many possibilities in which CFD can become such a useful tool for and
as long as the parameters are set up correctly, it is definitely a powerful tool in design.
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CFD MECH4406 Major Assignment Jack Lu 10426723
REFERENCES
Anderson, J.D. 1995, Computational fluid dynamics : the basics with applications, McGraw-Hill, New York.
ANSYS CFX 2005, ANSYS CFX 10.0 Help Files, ANSYS Inc.
O’Neill, P. & Rasnasinghe J. 2007, Computational Fluid Dynamics lectures, lecture notes taken from MECH4406 at The University of Western Australia School of Mechanical Engineering, Crawley in Semester 1 2007.
Ravinder, Y. & Prasad, N. 2006, Optimization of Intake System and Filter of an Automobile using CFD Analysis, MNR Filters, India
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